-list). Return non-nil if BUF1 should be added to the buffers tab. The
-default adds BUF1 if BUF1 and BUF2 have the same major mode, or if both
-match `buffers-tab-grouping-regexp'."
+list). The function should return non-nil if BUF1 should be added to the
+buffers tab. BUF1 will be omitted if any of the functions returns nil.
+
+Defaults to `select-buffers-tab-buffers-by-mode', which adds BUF1 if BUF1 and
+BUF2 have the same major mode, or both match `buffers-tab-grouping-regexp'."